Abstract This paper deals with an experimental investigation of comprehensive fracture toughness of syenite and basalt rocks by using a testing procedure which consists of three-point bending, asymmetric three-point bending and four-point shear tests. The results obtained show that the curve of rock fracture toughness of various modes can be approximated by an empirical quadratic equation, i.e. K I 2 +AK I K II +BK II 2 =K Ic 2 . The experimental data may be correlated with acoustic velocity in the form K Ic 2 =−1.146+0.540 V p , as well as with other mechanical parameters such as uniaxial compressive and tensile strength, modulus of elasticity, and so on. The observations from the tests show that the various characteristics of rock fracture such as the critical stress intensity factors, K Ic and K IIc , notch displacement, fracture toughness ratio K IIc /K Ic , and fracture angle θ are found to be dependent upon the petrological composition, grain size etc. Obvious decrease of fracture toughness for syenite rock with coarser grains implies that there is an influence of the mineral components, rock structure, the grain boundary cracks and intergranular cracks. From this point of view, the relation between the fracture toughness and the physico-mechanical properties of rocks may be understood through the characteristics of the rock microstructure.